• Prosthetic group features
• Thermodynamic properties
• Cytochrome b₅ in motif databases
• Cytochrome b₅ in alignment databases
• Cytochrome b₅ in 3­D databases
• References

Haem type	Haem iron coordination	Axial iron ligands	Formal iron oxidation/spin states
Haem b	Hexacoordinate	NHis; NHis	FeII (S=0); FeIII (S=1/2)

Cytochromes b (cyt b) can be defined as electron­transfer proteins having one or two haem b groups, noncovalently bound to the protein. The fifth haem iron ligand is always provided by a histidine residue. Cyt b possess a wide range of properties and function in a large number of different redox processes [1].

Cytochromes b₅ (cyt b₅) are ubiquitous electron transport proteins found in animals, plants and fungi [2]. The microsomal and mitochondrial variants are membrane­bound, while those from erythrocytes and other animal tissues are water­soluble [3, 4]. The family of cyt b₅­like proteins includes, besides cyt b₅ itself, haemoprotein domains covalently associated with other redox domains, in flavocytochrome b₂ (L­lactate dehydrogenase), sulphite oxidase, plant and fungal nitrate reductases [2], and plant and fungal cyt b₅/acyl lipid desaturase fusion proteins [5].

3­D structures of a number of cyts b₅ and yeast flavocytochrome b₂ are known [see references on structural studies of cyt b₅family]. The fold belongs to the +ß class, with two hydrophobic cores on each side of a ß­sheet. The larger hydrophobic core constitutes the haem­binding pocket, closed off on each side by a pair of helices connected by a turn. The smaller hydrophobic core may have only a structural role and is formed by spatially close N­terminal and C­terminal segments [2]. Histidines provide the fifth and sixth haem ligands, and the propionate edge of the haem group lies at the opening of the haem crevice (see Figure 3B5C). Two isomers of cyt b₅, referred to as the A (major) and B (minor) forms, have been identified which differ by a 180° rotation of the haem about an axis defined by the ­ and ­meso carbons [2, 6].

Thermodynamic properties of cytochrome b₅

Protein	ProTherm entry	Mutation	Method
Cytochrome b5; bovine (Bos taurus) liver	3107	wild type	thermal
	3108	F35Y	thermal
	3109	F35L	thermal
	3110	F35H	thermal
	3111	wild type	urea
	3112	F35Y	urea
	3113	F35L	urea

Cytochrome b₅ in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
CYTOCHROMEB5	PR00363	CYTOCHROME_B5	PS00191	BL00191

Cytochrome b₅ in alignment databases

Protein Superfamily	Protein Homology Domain	Pfam	LPFC 3­D alignment
00026; cytochrome b5 00071; L­lactate dehydrogenase 00132; nitrate reductase 00143; sulfite oxidase 03768; stearoyl­CoA desaturase	00006; cytochrome b5 core	PF00173; heme_1	-

PDB	scop	BSM	RELI Base	Header	¹
1aqa	1aqa	1aqa	1aqa	Cytochrome b5 (water­soluble domain) (reduced); rat (recombinant form expressed in Escherichia coli)	-
1aw3	1aw3	1aw3	1aw3	Cytochrome b5 (water­soluble domain) (oxidised); rat (recombinant form expressed in Escherichia coli)	-
1axx	1axx	1axx	-	Cytochrome b5 (water­soluble domain) (oxidised); rat (recombinant form expressed in Escherichia coli)	-
1b5a	1b5a	1b5a	1b5a	Cytochrome b5 form A (water­soluble domain); rat (recombinant form expressed in Escherichia coli)	-
1b5b	1b5b	1b5b	1b5b	Cytochrome b5 form B (water­soluble domain); rat (recombinant form expressed in Escherichia coli)	-
1b5m	1b5m	1b5m	1b5m	Cytochrome b5 (water­soluble domain); rat (outer mitochondrial membrane)	M7LBC124
1blv	-	1blv	1blv	Cytochrome b5 (oxidised); rat (recombinant form expressed in Escherichia coli)	-
1cyo	1cyo	1cyo	1cyo	Cytochrome b5 (oxidised); bovine (Bos taurus) liver	-
1fcb	1fcb	1fcb	1fcb	Flavocytochrome b2; Saccharomyces cerevisiae	MMS91044
1iet*	1iet*	1iet*	1iet*	Apocytochrome b5 (pH 6.2, 298 K); rat (expressed in Escherichia coli)	-
1ieu*	1ieu*	1ieu*	1ieu*	Apocytochrome b5 (pH 6.2, 298 K); rat (expressed in Escherichia coli)	-
1lco	1lco	1lco	1lco	Flavocytochrome b2 (complex with phenylpyruvate) (Y143F mutant); Saccharomyces cerevisiae	-
1ldc	1ldc	1ldc	1ldc	Flavocytochrome b2 (complex with pyruvate) (Y143F mutant); Saccharomyces cerevisiae	-
1ltd	1ltd	1ltd	1ltd	Flavocytochrome b2 (complex with sulphite); Saccharomyces cerevisiae	-
1wdb	1wdb	1wdb	1wdb	Cytochrome b5 (oxidised); bovine (Bos taurus) (expressed in Escherichia coli)	MS7RB4
3b5c	3b5c	3b5c	3b5c	Cytochrome b5 (oxidised); bovine (Bos taurus) liver	-

¹ Macromolecular Structures abstract. Full text is available to BioMedNet Members

References

1. Kiel, J.L. (1995) Type­b Cytochromes: Sensors and Switches. CRC Press, Boca Raton.
2. Lederer, F. (1994) The cytochrome b₅­fold: an adaptable module. Biochimie 76, 674-692.
3. Abe, K., Kimura, S., Kizawa, R., Anan, F.K. and Sugita, Y. (1985) Amino acid sequences of cytochrome b₅ from human, porcine, and bovine erythrocytes and comparison with liver microsomal cytochrome b₅. J. Biochem. (Tokyo) 97, 1659-1668.
4. Giordano, S.J. and Steggles, A.W. (1993) Differential expression of the mRNAs for the soluble and membrane­bound forms of rabbit cytochrome b₅. Biochim. Biophys. Acta 1172, 95-100.
5. Sperling, P., Schmidt, H. and Heinz, E. (1995) A cytochrome­b₅­containing fusion protein similar to plant acyl lipid desaturases. Eur. J. Biochem. 232, 798-805.
6. Rivera, M., Barillas­Mury, C., Christensen, K.A., Little, J.W., Wells, M.A. and Walker, F.A. (1992) Gene synthesis, bacterial expression, and ¹H NMR spectroscopic studies of the rat outer mitochondrial membrane cytochrome b₅. Biochemistry 31, 12233-12240.

Bibliography on structural studies of cytochromes b5

Reviews on cytochrome b5 family